A method wherein feed water is passed through ion exchange resins to effect deionization thereof has hitherto been known as a deionized water production process. In this method, however, the ion exchange resins, when exhausted with ions, must be regenerated with respective aqueous solutions of an acid and an alkali. In an effort to eliminate such a disadvantage involved in the foregoing treatment operation, an electrodeionization deionized water production process entirely without need of such regeneration with any chemicals has recently been established and put into practical use.
For this electrodeionization process, an ion exchange material(s) such as an ion exchange resin(s) or an ion exchange fiber(s) is filled between cation exchange membranes and anion exchange membranes to form ion depletion compartments, concentration compartments are provided on both outer sides of the ion depletion compartments, and the ion depletion compartments and the concentration compartments are disposed between an anode and a cathode. While applying a voltage, feed water is flowed into the ion depletion compartments and concentrating water is flowed into the concentration compartments. Impurity ions in the feed water are removed in the ion depletion compartments while electrically transferring the impurity ions to the concentration compartments to produce deionized water. Since the ion exchange material(s) is never saturated with ions according to this process, regeneration thereof with any chemicals is advantageously unnecessary.
However, the deionized water production process according to electrodeionization involves a problem of a low removal rate of silica in feed water. In order to solve this problem, the instant applicant has previously proposed electrodeionized water production equipment wherein ion exchange material layers through which feed water is first to be passed are anion exchange material layers (Japanese Patent Laid-Open No. 71,624/1992).
Although the silica removal rate can be improved using this equipment, the inventors of the present invention have made intensive investigations with a view to establishing a process for further increasing the silica removal rate. As a result, they have found out that the silica removal rate can be remarkably improved when the current direction of feed water being fed into ion depiction compartments is set opposite to the current direction of concentrating water being fed into concentration compartments while at the same time feed water flowed into the ion depletion compartments is first passed through respective anion exchange material layers. The present invention has been completed based on this finding.
An object of the present invention is to provide a process for producing deionized water by electrodeionization, wherein silica as an impurity in feed water can be removed at a very high removal rate.